The present invention relates to a circuit for driving a cold cathode fluorescent lamp (CCFL) and, in particular, the circuit for driving more than one CCFL.
A fluorescent lamp is broadly divided into a hot-cathode type and a cold-cathode type by constituent of the electrodes. In the cold cathode fluorescent lamp (CCFL), the electrodes consist of materials that radiate many electrons due to high voltages applied. In other words, the electrodes include no filament for thermionic emission, in contrast to the hot cathode fluorescent lamp. Thereby, the CCFL has the advantages over the hot cathode fluorescent lamp especially in its much smaller diameter tube, its longer life, and its lower power consumption. Because of these advantages, the CCFLs are extensively used for, in particular, the products strongly required to reduce the thickness (or size) and lower the power consumption, such as the backlights of liquid crystal displays and the light sources of FAXs and scanners.
The CCFL has, when compared to the hot cathode fluorescent lamp, the following electric characteristics: its breakdown voltage is higher; its discharge current (which is hereafter referred to as a lamp current) is smaller; and its impedance is higher. The CCFL has, in particular, the negative resistance characteristics, that is, its resistance value sharply drops with increase in lamp current.
The CCFL driver circuit is designed to match these electric characteristics of the CCFL. In particular, reduction in thickness (and size) of devices and lowering in power consumption are of importance in uses of the CCFLs, and accordingly, the CCFL driver circuit is strongly required to reduce its size (in particular, thickness) and lower its power consumption as well.
For example, the following is known as a conventional CCFL driver circuit. See, for example, Published Japanese patent application H08-273862 gazette. FIG. 12 is the circuit diagram showing the configuration of the conventional CCFL driver circuit. The conventional CCFL driver circuit comprises a high-frequency oscillator circuit 100, a step-up transformer T, and an impedance matching section 200. The high-frequency oscillator circuit 100 converts a direct voltage from a direct-current power supply DC into an alternating voltage of a high frequency, and applies the direct voltage across the primary winding L1 of the step-up transformer T. The step-up transformer T generates a voltage V across the secondary winding L2. The secondary voltage V is much higher than the primary voltage and applied across the CCFL FL through the impedance matching section 200. The impedance matching section 200 includes, for example, a series circuit of a choke coil L and a capacitor C. Here, the capacitor C includes stray capacitances around the CCFL FL. Impedance matching is achieved between the step-up transformer T and the CCFL FL by the adjustment with the inductance of the choke coil L and the capacity of the capacitor C.
A voltage is applied across the primary winding L1 of the transformer T when the CCFL FL stays out, and then, the voltage VR across the CCFL FL abruptly rises and exceeds the breakdown voltage because of the resonance between the choke coil L and the capacitor C in the impedance matching section 200. Thereby, the CCFL FL starts discharge and shining. Then, the resistance value of the CCFL FL sharply drops with increase in the lamp current IR, due to the negative resistance characteristics. Following that, the voltage VR across the CCFL FL falls. At that time, the impedance matching section 200 acts to maintain the stable lamp current IR, regardless of the changes in the voltage VR across the CCFL FL. In other words, the luminosity of the CCFL FL is maintained with stability.
The secondary winding L2 of the step-up transformer T and the choke coil L are represented as separate circuit elements in FIG. 12. In the actual CCFL driver circuit, however, the secondary winding of a leakage transformer performs three functions; step-up, choke, and impedance-matching functions, as follows. FIG. 13 is a perspective view schematically showing the appearance of the leakage transformer T used in the conventional CCFL driver circuit as a transformer for the power supply. FIG. 14 is a cross-sectional view of the leakage transformer T taken along a line XIV—XIV shown in FIG. 13. The arrows shown in FIG. 13 represent the eye direction. In the leakage transformer T, the primary winding L1 and the secondary winding L2 are wound around the rod-shaped core CR so as to be located adjacent to each other. Here, a first partition D1 is provided between the primary winding L1 and the secondary winding L2 to prevent electric discharge between the both windings. Similarly, a plurality of second partition D2 divides the secondary winding L2 to reduce stray capacitances between the lines of the winding, while preventing electric discharge between the lines of the winding. The stray capacitances are hereafter referred to as line-to-line capacitances. A split winding refers to such a winding of a width divided by partitions. The step-up ratio of the leakage transformer T depends on a turn ratio between the primary winding L1 and the secondary winding L2. Since the step-up ratio is high, in general, the turns of the secondary winding L2 are larger in number than the turns of the primary winding L1. Accordingly, the secondary winding L2 is larger in width than the primary winding L1, in general. In the leakage transformer T, in addition, the primary winding L1 and the secondary winding L2 are wound around the rod-shaped core CR and located adjacent to each other. Therefore, the leakage transformer T has the large leakage flux, and thereby, has the high output impedance. The inductive component of this high output impedance, that is, the leakage inductance resonates with the capacitor C, and acts as the above-described choke coil L. See FIG. 12. In the leakage transformer T, furthermore, the above-described leakage inductance and the line-to-line capacitance of the secondary winding L2 are easily adjusted. Accordingly, the impedance matching section 200 is easily composed of the secondary winding L2 and the above-described capacitor C.
The leakage transformer T is easily designed as described above, and in particular, the secondary winding L2 can be used as the above-described choke coil L. Accordingly, for the conventional CCFL driver circuits, leakage transformers are considered to have advantages especially in miniaturization, and therefore, extensively used.
The backlights of liquid crystal display, in particular, require high luminosity. Accordingly, the installation of more than one CCFL is desirable for use as the backlights. At that time, the luminosity must be made uniform among those CCFLs. Furthermore, the CCFL driver circuit in miniature size is necessary. Parallel driving of those CCFLs by a common power supply is desirable in order to meet those requirements.
However, the parallel driving is difficult for the following reason: CCFLs have negative resistance characteristics as described above. Accordingly, just in a parallel connection of more than one CCFL, the currents are concentrated into only one CCFL, and after all, only one CCFL can shine. Furthermore, when many CCFLs are connected to a common power supply, the wiring arrangements between the CCFLs and the power supply are different from each other, in particular, in wire length. Accordingly, the stray capacitances vary among the CCFLS. Therefore, in the parallel driving of more than one CCFL, each lamp current of the CCFLs should be separately controlled, and thereby, variations of the lamp currents be suppressed. It is difficult to achieve all the following: using a leakage transformer as a common choke coil among more than one CCFL, matching the impedance of the leakage transformer to the impedance of each CCFL, and controlling each lamp current with high precision. Here, the difficulty is similar when a piezoelectric transformer is used instead of the leakage transformer.
Therefore, the conventional CCFL driver circuit is equipped with one power supply (in particular, a leakage transformer) for each CCFL, and makes each power supply control a lamp current to be uniform. In other words, the power supplies as many as the CCFLs are necessary for the conventional CCFL driver circuit. As a result, reduction in component count is difficult, and thereby, further miniaturization of the whole device is difficult.